RDMA/efa: Set maximum pkeys device attribute

The max_pkeys device attribute was not set in query device verb, set it to
one in order to account for the default pkey (0xffff). This information is
exposed to userspace and can cause malfunction

Fixes: 40909f66

 ("RDMA/efa: Add EFA verbs implementation")
Link: https://lore.kernel.org/r/20200614103534.88060-1-galpress@amazon.com
Reviewed-by: Firas JahJah <firasj@amazon.com>
Reviewed-by: Yossi Leybovich <sleybo@amazon.com>
Signed-off-by: Gal Pressman <galpress@amazon.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>